stiffen
Concise Oxford English Dictionary © 2008 Oxford University Press:
stiffen/ˈstɪfn/
▶verb
- 1 make or become stiff.
- 2 make or become stronger or more steadfast.
– derivatives
stiffener noun,
stiffening noun.
stiffener noun,
stiffening noun.
'stiffen' also found in these Oxford entries:

